In drilling a deep oil well, it is normally intended that the well be vertical. State regulatory agencies define a vertical well by permitting little drift from the vertical, typically up to about 3.degree.. To be sure, deviated wells are intentionally drilled in many instances, but this is always done in accordance with special permits. It is also done is a more tedious fashion, typically being carried out with a directional tool and drilling motor. The more conventional fashion of drilling a well, even the straight portions of wells ultimately deviated, is all carried out by drilling vertically downwardly. Maximum rate of penetration is an important measure of the efficiency of the drilling process. If the process becomes slow, the time required for drilling increases, and the cost of the well ultimately increases. This is undesirable. With this straight hole drilling tool, the operator is able to maintain the weight on bit, hence, maintain a higher penetration rate.
The rate of drilling is improved some by drilling vertically. Moreover, the rate of drilling is improved by drilling with as true a vertical well as can be possibly accomplished. Generally, more weight on the bit increases the rate of penetration, ROP below. The ROP is reduced where the well drifts from the vertical and weight on bit is decreased. In part, this reduction results from scuffing of the drill string against the side of the previously drilled borehole. Scuffing occurs and sometimes becomes so extreme that the hole is distorted from a round hole. The typical rotary drill bit forms a round hole. After the round hole has been formed to any reasonable depth, deviated drilling therebelow may prompt the drill string to rub against the side wall and thereby scuff the side wall forming a non-round portion. This is known as a key seat. The key seat is an area of trouble. It absorbs the energy required for rotation of the drill bit. In addition, it also raises the risk of tool failure or differential pressure sticking. This may hold the drill string against the side wall so that it is impossible to proceed any further. When that occurs, remedial steps have to be undertaken. One of the remedial steps is pulling the drill string, and then drilling with a reamer, mud motor, or hole straightener tool to reduce deviation.
Deviation of the borehole from a vertical plumb line is inevitable because the rotation of the drill bit sets Up a gyroscopic precessional drift, and irregularities in the resistance of the formations drilled will also prompt some measure of curvature. (Many bits have offset cutters). Also, bits are made with eccentric cutters. Most do riot consider the dip of earth formations encountered by the drill bit. If, for instance, several strata are encountered which are alternately hard and soft strata and the formation has a dip to the east of 45.degree., there is a tendency of the drill bit to slide or skid as it encounters each soft/hard interface. There are other causes of drift. Ultimately, all of these occur notwithstanding the fact that the drill string is formed of metal tubular members which, on the surface, are quite straight.
A typical drill stem is formed by assembling (from the hit up) several drill collars for weight and then a string of drill pipe. The drill stem is technically the entire assembled system above the drill bit including the string of drill pipe and drill collars. The drill pipe is typically conventional wall drill pipe which is somewhat flexible. The flexibility ol the drill pipe comes into play when drilling several thousand feet. Drill collars are added between the drill string and the drill bit to stiffen the lower portion on the theory that the stiffness of the drill collars provides better directional tracking. Also, the drill collars raise the weight on the bit (WOB hereinafter). Whether tracking ultimately is true depends on a number of dynamically encountered conditions including formation dip, precessional drift bit walking and others factors. The stiffness of the drill collars is significantly greater than the stiffness of the individual pipe joints making up the drill string. Drill collars are a joint of pipe formed with an extra thick pipe wall so that weight on the bit and drill collar stiffness are significantly increased. In simple language, it is hard to bend a drill collar because it is extra thick. Generally, drill collars have a thick wall, and are round or cylindrical shape. To be sure, in about 1965, several patents were issued to Fred K. Fox and many others relating to externally grooved drill collars. Still, such pipe were generally axially symmetrical in the sense that the mass moment of inertia was kept centered. Shallow helical grooves, checker board flats and other flats formed on the exterior did not significantly change the mass moment of inertia from the center-line axial position. When rotated in the conventional manner, there is no unwanted vibration or other wobble induced by variations from a traditional cylindrical, smooth wall construction. With or without helical turns or other flow paths along the exterior, drill collars are generally constructed so that the mass moment of inertia is aligned with the centerline. In this disclosure, a drill collar is set forth where the center of mass and the mass moment of inertia are offset from the centerline axis. In other words, the device is eccentered so that certain desirable vibrations are set up in operation.
The drill collar of the present disclosure sets up a wobbling vibration in operation. As a practical matter, at typical drilling, rates the several drill collars will wobble about a defined centerline axis. If the drill stem is being rotated at 50 rpm or more in a well borehole that is precisely vertical without drift, some wobble will occur. Assume further that the drill bit drills a hole in excess of 9" diameter and that the drill stem is made up primarily of drill pipe with a diameter of 5". This defines an annular gap of about 2" on the exterior for the annular space between the drill pipe and the open hole. As the pipe string becomes longer, there is a tendency for the column of pipe to rotate at the required velocity with some wobble along the length of the column. Imperfections in the pipe string including non-round pipe, bent joints and the like will prompt some wobble or rotation from the centerline axis. This wobble will materially impact the entire column of pipe. To be sure, wobble is generally suppressed in the area where the drill collars are located because they are much stiffer and have a reduced tendency to wobble from the centerline axis of the drill stem. Even so, there is some modest amount which is so small as to be imperceptible in that region.
The present disclosure takes advantage of eccentering of the drill collar. More than that, it takes advantage of cutters which are attached at specified locations along the drill collar so that reaming of other narrow areas is accomplished thereby maintaining a straighter and more true borehole. Preferably, the drill collar of the present disclosure is conveniently formed of a square billet which is drilled through to define a drill collar. If perfectly square, it will provide a centered mass and a mass moment of inertia coincident with the centerline axis. In this particular instance, it is constructed of a square billet with three full gauge corners in the cross-sectional shape. The fourth corner is modified, i.e., it is removed. This provides an offset or eccentering of the mass moment of inertia and center of gravity. Moreover, this prompts the several drill collars connected in a string of drill pipe to create a region of wobble which extends beyond its normal location. If, for instance, the drill collar has a nominal diametric measure of 7", this wobble might effectively create a locus of movement which has a diameter of about 7.5". That enables an eccentered cutter to ream part of the borehole away and thereby making at more true borehole.
While the foregoing describes some benefits of the device, it is summarized as an eccentered drill collar formed of a square billet having three parallel square edges along the length thereof. The fourth edge is reduced so that the mass moment of inertia is eccentered to thereby induce controlled vibrations. Moreover, the controlled vibrations enable one or more cutters along the length to wobble into contact with the low sided borehole, thereby reaming the borehole to maintain or restore the borehole to reduce deviation of the well borehole. The borehole remains relatively true after being reamed through this drill collar equipped with cutters which cuts to a more true circle with ;i more true centerline axis free of drift.